I don't see anything in the rulebook that specifically disallows extended magwells. But there is a section that says the rifle/handgun must "be of a factory configuration". This could be interpreted really widely or really narrowly depending on the match. Anyone know if magwells are included under this section or not?
I have never been to a match that did not allow magwells, but it does make me wonder why this rule is there. I have seen it in several rulebooks. It is either never enforced or it means something other than how I read it.